LEDs emit light of a single color. It is common practice to create white light using a blue LED and irradiating one or more phosphors. The phosphor may add red and green components or may add a yellow-green component (e.g., a YAG phosphor). Such techniques include depositing the phosphor(s) in a reflective cup surrounding the blue LED, or mixing the phosphors in a bonding medium (e.g., silicone) and depositing the mixture on the blue LED.
Using a yellow-green phosphor with a blue LED produces a high temperature white light that is not visually pleasing and has poor color rendering. Adding a red phosphor makes the light warmer.
Providing the phosphors in a reflector cup surrounding the LED results in a relatively large light source. Further, the thickness of the phosphor/binder material in the cup varies, making it difficult to achieve a desired color temperature and uniform color. Further, the walls of the cup block the side-light, which limits the possible emission patterns. Other drawbacks exist.
When depositing two or more phosphors in conjunction, it is difficult to create a predictable and consistent light color, and the different phosphors may react differently during the same deposition process. For example, the relative phosphor thicknesses and densities are difficult to control, the light conversion of one phosphor has an effect on the light conversion of the other phosphor, and different phosphors typically have different physical properties (e.g., density differences and temperature coefficients of expansion) and chemical properties so do not react the same when undergoing the same deposition process.
What is needed is a technique for generating white light using phosphors and a blue, UV, or near UV LED that is relatively easy to implement and consistently produces the desired white light temperature. It is particularly desirable to generate a warm white light (more red) in the temperature range of 2000K-5000K.